Altered patterns of senescence and ripening in gf, a stay-green mutant of tomato (Lycopersicon esculentum Mill.)

The gf tomato mutant, which retains chlorophyll during ripening, has been f ound to be affected in leaf senescence. The leaves of the gf mutant show an absolute stay-green phenotype. As leaf senescence and fruit ripening proce ed, there is a marked difference in chlorophyll content between wild-type a nd gf. In both attached and detached leaf studies, or after treatment with ethylene, the leaves withered and abscised in gf with only slight loss of c hlorophyll and carotenoids. Total protein content declined and free amino a cids increased during leaf senescence in wild-type and gf, but Western anal ysis showed that LHCII polypeptides were retained at higher levels in gf. E xpression of senescence-related mRNAs increased normally in gf whereas thos e for cab, rbcS and rbcL declined in both mutant and wild-type. The mutant possesses enzyme activity for chlorophyllase, the formation of phaeo-phorbi de a by the action of Mg-dechelatase and the oxygenolytic opening of the po rphyrin macrocycle. Analysis of chlorophyll breakdown products in fruit ind icated that gf, like other stay-green mutants, accumulates chlorophyllides a and b, but phaeophorbide a does not accumulate in vivo. This may indicate that, in the mutant, in vivo the action of phaeophorbide a-oxygenase is so mehow prevented, either by altered accessibility or transport of components required for thylakoid disassembly or the absence of another factor.